The invention relates to a method for capacitively measuring the thickness of multi-layer films, the dielectric constants of the layers of which differ at least at a particular temperature.
It is well known that the thickness of plastic films can be measured capacitively, because the capacitance of a measuring capacitor, through the electric field of which the film passes, is affected by the thickness of the film and by the dielectric constant of the film material. The results of such thickness measurements are used in the production of plastic films, for example, in the production of tubular film to control the thickness of the film in a closed control circuit, so that high-grade film with a defined thickness, which is essentially constant over the whole width, is obtained.
Since it is difficult in tubular film plants to dispose one of the capacitor plates of the measuring capacitor in the interior of the tubular film, only the edge field of the measuring capacitor is utilized for measurements at the tubular film. In this case, both capacitor plates of the measuring capacitor are disposed outside of the tubular film and oriented at right angles to the plane of the film. On the other hand, if the film web is placed flat, it is also possible to measure the thickness capacitively with the help of a measuring capacitor, the capacitor plates of which are disposed on opposite sides of the film web. Alternatively, a mechanical measuring method can also be used in this case. In every case, the value, obtained for these measurements, is a measure of the total thickness of the film.
For packaging purposes, however, multi-layer films are frequently used, which have a two-layer or multi-layer construction. Packaging films, which are used, for example, to keep food fresh, must not only be sealable and impermeable to water vapor, but also impermeable to air and especially to oxygen. This can be achieved only by multi-layer films, since a single film material by itself does not satisfy all requirements. For example, films of polyethylene (PE) can be sealed and are impermeable to water vapor but not to oxygen. This is the case for low density polyethylene (LDPE), linear polyethylene of low density (LLDPE) as well as high density polyethylene (HDPE). On the other hand, film materials such as polyamide (PA) and ethylene vinyl alcohol (EVOH) are impermeable to oxygen (EVOH only in a dry medium), but cannot be sealed. A frequently used layer construction for packaging films therefore consists of a middle layer of EVOH, which is protected against the effects of water vapor by sealable LDPE layers, which are applied on both sides. However, a bonding agent is inserted between the various layers, so that adhesion of the layers is brought about.
The middle layer of relatively expensive EVOH should be as thin as possible, but not fall below a minimum thickness, so that the impermeability to oxygen remains assured.
It is an object of the invention to indicate a measuring method, which permits not only the total thickness, but also the thickness of individual layers of a multi-layer film to be determined.
This objective is accomplished in a method of the type named above owing to the fact that, in addition to the capacitive measurement, at least one further thickness measurement is carried under different conditions and that the thicknesses of the individual layers are determined by comparing the results of the measurements and by means of the different dielectric constants.
The invention makes use of the fact that many plastic materials, which are frequently used as a barrier layer for oxygen in multi-layer films, have dielectric constants, which are clearly temperature dependent. Other plastic materials, such as PE, have a dielectric constant, which is almost constant or decreases slightly with increasing temperature up to about 110xc2x0 C. On the other hand, dielectric constants of materials, such as EVOH or PA, increase clearly above a particular critical temperature. For this reason, the capacitive measurement of the thickness of multi-layer films, which contain such material, had to be carried out at a relatively low temperature until now, that is, generally only after the films have been collapsed, as otherwise the temperature dependence of the dielectric constants would lead to a distortion of the measurement results. However, if two capacitive thickness measurements are carried out at different temperatures, different measurement results are obtained and this difference is a measure of the ratio of the thickness EVOH layer to the total thickness of the film. Since the dielectric constants of the participating materials at the two different measurement temperatures are known, the proportions of the thicknesses of the different materials can be determined quantitatively.
Alternatively, instead of two capacitive measurements at different temperatures, it is also possible to combine a mechanical measurement of the total thickness of the film with a capacitive measurement at a temperature, at which the dielectric constants of the two layer materials differ clearly from one another.
In a preferred embodiment of the invention, a first capacitive measurement during the manufacture of the tubular film is carried out at a relatively high temperature directly on the tubular film and a second capacitive or mechanical measurement takes place after the film has been collapsed at an appropriate, lower temperature.